It is more exactly like a smell of old lubricant oil heated by the hot engine block. Like I mentioned in another reply, my old 300 SD has a lot of blow back from an old engine and this blow back have a typical smell of burned oil.
When I stop on traffic lights with my A/C system running in ventilation only I get this typical smell inside the cabin. It is not as a smell of exhaust system but as a smell of heated lube oil.
I had no chance yet to verify what is around the A/C air filter.
Was my explanation enough?
When I asked about the carbon filter I do not mean that I want the filter to solve that problem but to learn better about the carbon activated filters.
Thanks

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Nyck - TX - USA

Now we're getting somewhere. The smell of burnt oil is different from the smell of exhaust. It's entirely possible you have an oil leak somewhere and you're smelling the hot oil on the block. Would be worth your time to go peeking under and around the engine with a strong flashlight and see if you've got some oil leaking somewhere. Under-hood oil leaks that get hot enough WILL cause the smell to come in through the fresh-air intake since it's right there at the hood.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Yes, it is exactly what you are saying. That is the smell that I am getting in my cabin. Thanks for your help.

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Nyck - TX - USA